The present invention relates to a method for moving workpieces in a vacuum chamber for high processing throughput. More specifically, it involves a method which maintains a nearly continuous flow of workpieces for processing as they are introduced into a vacuum chamber, aligned to a suitable orientation, processed one at a time, and removed from the vacuum chamber.
During the processing of silicon wafers in the manufacture of microelectronic circuits, up to twenty different types of tools are employed for effecting several hundred processing steps. Most of these processing steps must be performed in a vacuum chamber at pressures less than 1.times.10.sup.-3 torr, and each requires from ten seconds to three minutes per wafer. Most of the processing tools operate on wafers one at a time in order to optimize control and reproducibility in a manufacturing environment.
In general, each operation on a wafer must be performed in a particular order, so that each operation must wait until rite completion of a preceding one, and, in turn, affects the time a wafer is available for a subsequent step. Tool productivity or throughput for vacuum processes that are relatively short, such as ion implantation, can be severely limited if the work flow to the processing location or platen is interrupted by sequential events, which may include, for example, the introduction of the wafer into the vacuum system, the transfer of a wafer in the vacuum chamber or the exchange of wafer carriers or cassettes.
In order to increase throughput, it is desirable to shorten the duration of sequential events, i.e., those events which must be performed consecutively. For this reason, the pumpdown times (to high vacuum) and the venting times (to atmospheric pressure) are usually accomplished as quickly as possible, in less than about a half minute. However, high levels of foreign material or particles carried deep into the process line may result from the turbulence of such rapid cycling. These particles become distributed on the surface of the silicon wafer and result in more defects for a higher fraction of the micro-electronic circuits being fabricated, reducing manufacturing yields significantly.
The prior art has sought to address these concerns in a number of ways. Early systems employed wafer handling stations that introduced wafers into the vacuum process chamber through a load lock one at a time by mean of a gravitation slide. Each wafer was processed, and exited the vacuum process chamber through a second load lock as shown, for example, in FIG. 1 of U.S. Pat. No. 4,282,924. These systems suffered from several problems as wafer sizes employed in micro-electronic device fabrication evolved to larger diameters of six and eight inches, including wafer breakage and high levels of defect generation resulting from both sliding and rapid vacuum cycling. Also, manufacturers tracked work in process through the factory by tracking the progress of the wafer carriers, not the individual wafers, through the line, and since wafers were not returned to their original carrier or cassette, this complicated tracking of the work in process through the factory.
Another prior art approach used a wafer handling station in which the two cassettes are introduced, one each through load locks #1 and #2, and are pumped down simultaneously. Wafers were processed in an alternating manner from both load locks; for example, the first wafer from load lock #1, the second from load lock #2 and so on until all wafers from both load locks had been processed. In this system, wafers are returned to their original cassette, however the pumpdown and venting to atmosphere are sequential to the processing. Also this scheme requires that both cassettes of wafers be introduced to or removed from the load lock chambers in essentially the same time interval while processing is halted. Since production lots may very often be only a single cassette of wafers, this is not always possible. Another problem with this system is the requirement that the operator be present immediately following processing to exchange the processed cassettes of wafers for the next cassettes waiting to be processed, in order not to lose valuable process time.
It would be desirable to arrange the handling or movement of wafers in a manner which assured a continuous flow of wafers from cassettes which have been transferred into the vacuum, while increasing time available for slowly venting the load locks, exchanging the cassettes of processed wafers for unprocessed ones, and slowly pumping the load locks containing the cassettes to a suitable vacuum.
It would also be desirable to effect wafer handling with a small number of motive mechanisms while enhancing the flow of wafers.